1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for transmitting a frame in a wireless communication system including a relay station.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4th generation) mobile communication standard following the IMT-2000. It was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading, and an interval of a transmitted symbol is increased, thereby minimizing inter-symbol interference. When a system uses the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the plurality of users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner.
In an OFDMA system, frequency diversity for multiple users can be obtained by using frequency selective scheduling, and subcarriers can be allocated variously according to a permutation rule for the subcarriers. In addition, a spatial multiplexing scheme using multiple antennas can be used to increase efficiency of a spatial domain. In order to support the various schemes described above, a control signal must be transmitted between a mobile station (MS) and a base station (BS). Examples of the control signal include a channel quality indicator (CQI) for reporting a channel state from the MS to the BS, an acknowledgement/not-acknowledgement (ACK/NACK) signal in response to data transmission, a bandwidth request signal for requesting allocation of a radio resource, precoding information in a multiple antenna system, antenna information, etc. The control signal is transmitted through a control channel.
A wireless communication system including a relay station (RS) has recently been developed. The RS is employed for cell coverage extension and transmission capability improvement. A BS provides a service to an MS located in a coverage boundary of the BS via the RS, and thus can obtain an effect of extending the cell coverage. In addition, the RS improves signal transmission reliability between the BS and the MS, thereby improving transmission capacity. The RS can be used when the MS is located in a shadow area even if the MS is located within the coverage of the BS.
A midamble is a signal transmitted by the BS to allow the MS to directly measure a channel state. When the BS transmits signals by using a multiple input multiple output (MIMO) technique through a plurality of antennas, the BS can transmit different signals for the respective antennas or transmit signals at different locations in a resource region, and the MS can measure a channel state for each antenna of the BS by receiving a midamble and thus estimate a channel state of a serving cell or an interference level of a neighbor cell. The BS can adaptively schedule resources by receiving a feedback of the channel state estimated by the MS.
The wireless communication system including the RS requires a new frame structure different from the conventional frame structure. In addition, similarly to a case where the MS receives a preamble or a midamble to establish synchronization with the BS or to estimate a channel state, the RS also needs to receive a signal to establish synchronization with the BS or to estimate the channel state.